U.S. patent application number 12/006833 was filed with the patent office on 2008-07-31 for brake system.
This patent application is currently assigned to Honda Motor Co., Ltd.. Invention is credited to Satoshi Matsushita.
Application Number | 20080179941 12/006833 |
Document ID | / |
Family ID | 39271115 |
Filed Date | 2008-07-31 |
United States Patent
Application |
20080179941 |
Kind Code |
A1 |
Matsushita; Satoshi |
July 31, 2008 |
Brake system
Abstract
A brake system includes an electric brake force generator which
brakes a wheel using a driving force of an electric motor and an
electric motor controller that performs a field-weakening control
of the electric motor. The electric motor controller performs the
field-weakening control of the electric motor which then operates
the electric brake force generator. Thus, the rotational speed of
the electric motor is increased when the field weakening control is
performed by the electric motor controller thereby quickly
activating the electric braking force generator to enhance response
of brake force generation.
Inventors: |
Matsushita; Satoshi;
(Saitama, JP) |
Correspondence
Address: |
CARRIER BLACKMAN AND ASSOCIATES
24101 NOVI ROAD, SUITE 100
NOVI
MI
48375
US
|
Assignee: |
Honda Motor Co., Ltd.
Tokyo
JP
|
Family ID: |
39271115 |
Appl. No.: |
12/006833 |
Filed: |
January 4, 2008 |
Current U.S.
Class: |
303/20 |
Current CPC
Class: |
B60T 13/745 20130101;
B60T 2201/03 20130101; B60T 8/3275 20130101; B60T 8/4081 20130101;
B60T 13/662 20130101; B60T 17/221 20130101 |
Class at
Publication: |
303/20 |
International
Class: |
B60T 13/00 20060101
B60T013/00 |
Foreign Application Data
Date |
Code |
Application Number |
Jan 30, 2007 |
JP |
2007-19753 |
Claims
1. A brake system comprising: an electric braking force generator
that brakes a wheel by a driving force of an electric motor; and an
electric motor controller that performs a field-weakening control
of the electric motor.
2. The brake system according to claim 1, wherein the electric
motor controller sets a target braking force in accordance with at
least an operational amount of braking operation by a driver, and
performs the field-weakening control in accordance with a rate of
change of the target braking force.
3. The brake system according to claim 1, wherein the electric
motor controller detects load characteristics of the electric
braking force generator, and performs the field-weakening control
in accordance with the load characteristics.
4. The brake system according to claim 3, wherein the electric
motor controller detects the load characteristics each time an
ignition switch of a vehicle on which the brake system is installed
is turned on.
5. The brake system according to claim 1, wherein the electric
motor controller detects load characteristics of the electric
braking force generator, sets a target braking force in accordance
with an operational amount of braking operation by a driver, and
performs the field-weakening control based on the detected load
characteristics and a rate of change of the target braking
force.
6. The brake system according to claim 2, wherein the electric
motor controller sets the target braking force and performs the
field-weakening control in accordance with the rate of change of
the target braking force when the rate of change exceeds a
predetermined value.
7. The brake system according to claim 3, further comprising a
brake caliper driven by the electric braking force generator,
wherein the electric braking force generator comprises a slave
cylinder having a piston, and wherein the electric motor controller
detects the load characteristics based on fluid pressure of the
brake caliper and at least one of a rotational position of the
motor and an advance position of the slave cylinder piston.
8. A brake system comprising: an electric braking force generator
including a slave cylinder and an electric motor which drives the
slave cylinder to generate a brake fluid pressure which brakes a
wheel; a brake caliper which receives brake fluid pressure from
said slave cylinder; and an electric motor controller that performs
a field-weakening control of the electric motor; wherein the
electric motor generates a torque sufficient to brake a vehicle
under normal braking conditions, and the controller performs the
field-weakening control of the motor to increase the rotational
speed of the motor under abnormal braking conditions.
9. The brake system according to claim 8, wherein the electric
motor controller sets a target braking force in accordance with at
least an operational amount of braking operation by a driver,
determines that an abnormal braking condition exists when a rate of
change of the target braking force exceeds a predetermined value,
and performs the field-weakening control in accordance with the
rate of change of the target braking force.
10. The brake system according to claim 8, wherein the electric
motor controller detects load characteristics of the electric
braking force generator, and performs the field-weakening control
additionally based on the detected load characteristics.
11. The brake system according to claim 10, wherein the electric
motor controller detects the load characteristics each time an
ignition switch of a vehicle on which the brake system is installed
is turned on.
12. The brake system according to claim 10, wherein the slave
cylinder has a piston, and the electric motor controller detects
the load characteristics based on fluid pressure of the brake
caliper and at least one of a rotational position of the motor and
an advance position of the slave cylinder piston.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] The present invention claims priority under 35 USC .sctn.119
based on Japanese patent application No. 2007-19753 filed 30 Jan.
2007. The subject matter of this priority document is incorporated
by reference herein.
BACKGROUND OF THE INVENTION
[0002] 1. Field of the Invention
[0003] The present invention relates to a brake system comprising
an electric braking force generator that brakes a wheel by using a
driving force of an electric motor.
[0004] 2. Description of the Related Art
[0005] Japanese Patent Application Laid-open No. 2005-343366
discloses a brake system of the type referred to as a brake by wire
(BBW) brake system, which converts a braking operation of a driver
into an electrical signal used to operate a motor cylinder or slave
cylinder as an electric braking force generator, and operates a
wheel cylinder by brake fluid pressure generated by the motor
cylinder.
[0006] In an emergency situation such as collision avoidance, the
rotational speed of the electric motor of the slave cylinder is
required to be increased so as to quickly build-up the brake fluid
pressure. However, if the rated rotational speed of the electric
motor is set at a high value in order to enhance the response of
braking force generation in an emergency situation, the rated
torque decreases correspondingly, leading to a possibility that the
torque of the electric motor becomes insufficient under normal
operating conditions. If both the rated rotational speed and the
rated torque of the electric motor are set at high values against
this possibility, a problem occurs in that the size and/or price of
the electric motor are increased.
SUMMARY OF THE INVENTION
[0007] The present invention has been made in view of the
above-described circumstances, and it is an object of the present
invention to enhance the response of braking force generation
without increasing the size of an electric motor that drives an
electric braking force generator.
[0008] To achieve the above object, according to a first aspect of
the present invention, there is provided a brake system comprising:
an electric braking force generator that brakes a wheel using a
driving force of an electric motor; and an electric motor
controller that performs a field-weakening control of the electric
motor.
[0009] With the first aspect of the present invention, the electric
motor controller performs the field-weakening control of the
electric motor which operates the electric braking force generator.
Therefore, while the electric motor generates a torque which is
sufficient under normal operating conditions, the rotational speed
of the electric motor is increased when the field-weakening control
is performed by the electric motor controller, thereby quickly
activating the electric braking force generator to enhance response
of braking force generation.
[0010] According to a second aspect of the present invention, in
addition to the first aspect, the electric motor controller sets a
target braking force in accordance with at least an operational
amount of braking by a driver, and performs the field-weakening
control in accordance with a rate of change in the target braking
force.
[0011] With the second aspect of the present invention, the
electric motor controller sets the target braking force in
accordance with the operational amount of braking by the driver,
and performs the field-weakening control in accordance with the
rate of change in the target braking force. Therefore, in an
emergency where a quick build-up of the brake fluid pressure is
needed, it is possible to increase the rotational speed of the
electric motor to quickly activate the electric braking force
generator.
[0012] According to a third aspect of the present invention, in
addition to the first aspect, the electric motor controller detects
load characteristics of the electric braking force generator, and
performs the field-weakening control in accordance with the load
characteristics.
[0013] With the third aspect of the present invention, the electric
motor controller detects the load characteristics of the electric
braking force generator, and performs the field-weakening control
in accordance with the load characteristics. Therefore, the
increase in the rotational speed of the electric motor can
compensate for a delay in the response of braking force generation
due to a difference in the load characteristics of the electric
braking force generator.
[0014] According to a fourth aspect of the present invention, in
addition to the third aspect, the electric motor controller detects
the load characteristics when an ignition switch is turned on.
[0015] With the fourth aspect of the present invention, the
electric motor controller detects the load characteristics of the
electric braking force generator when the ignition switch is turned
on. Therefore, it is possible to perform an appropriate
field-weakening control by detecting the latest load
characteristics each time the engine is started.
[0016] Although the following disclosure offered for public
dissemination is detailed to ensure adequacy and aid in
understanding of the invention, this is not intended to prejudice
that purpose of a patent which is to cover each new inventive
concept therein no matter how it may later be disguised by
variations in form or additions of further improvements. The claims
at the end hereof are the chief aid toward this purpose, as it is
these that meet the requirement of pointing out the improvements,
combinations and methods in which the inventive concepts are
found.
[0017] There have been chosen specific embodiments of a brake
system according to the invention and specific alternative
structures and modifications thereto, the embodiments chosen for
the purposes of illustration and description of the structure and
method of the invention are shown in the accompanying drawings
forming a part of the specification.
BRIEF DESCRIPTION OF THE DRAWINGS
[0018] FIGS. 1 to 6 show a first embodiment of the present
invention wherein
[0019] FIG. 1 is a fluid-pressure circuit diagram of a vehicle
brake system according to a first embodiment when the vehicle is
operating under normal conditions;
[0020] FIG. 2 is a fluid-pressure circuit diagram of a vehicle
corresponding to FIG. 1 when the brake system is operating under
abnormal conditions;
[0021] FIG. 3 is a block diagram of a control system of the vehicle
brake system according to the first embodiment;
[0022] FIG. 4 is a block diagram of a field-weakening control
section of an electronic control unit according to the first
embodiment;
[0023] FIG. 5 is a graph showing a map used for determining a
magnetic field current command value based on a rate of change in a
target fluid pressure according to the first embodiment; and
[0024] FIG. 6 is a graph showing a relationship between rotational
speed and torque when changing a field current command value Iq* of
a q-axis component of the electric motor.
[0025] FIGS. 7A to 10 show a second embodiment of the present
invention wherein
[0026] FIGS. 7A to 7C are diagrams or graphs showing a technique of
enhancing the response by a field-weakening control according to
the second embodiment;
[0027] FIG. 8 is a flowchart showing the field-weakening control
according to the second embodiment;
[0028] FIG. 9 is a graph showing a technique of detecting load
characteristics of a slave cylinder according to the second
embodiment; and
[0029] FIG. 10 is a graph showing a map used for determining a
magnetic field current command value based on a motor rotational
position according to the second embodiment.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0030] As shown in FIG. 1, a tandem master cylinder 11 has two
fluid pressure chambers 13A and 13B which output brake fluid
pressure according to a pushing force applied to a brake pedal 12
by a driver treading on the brake pedal 12. One of the fluid
pressure chambers 13A is connected to wheel cylinders 16 and 17 of
disc brake devices 14 and 15 for braking, for example, a left front
wheel and a right rear wheel through fluid passages Pa, Pb, Pc, Pd,
and Pe. The other fluid pressure chamber 13B is connected to wheel
cylinders 20 and 21 of disc brake devices 18 and 19 for braking,
for example, a right front wheel and a left rear wheel through
fluid passages Qa, Qb, Qc, Qd, and Qe.
[0031] A shutoff valve 22A, which is a normally open solenoid
valve, is provided between the fluid passages Pa and Pb. A shutoff
valve 22B, which is a normally open solenoid valve, is provided
between the fluid passages Qa and Qb. A slave cylinder 23 is
provided between the fluid passages Pb, Qb and the fluid passages
Pc, Qc. An anti-lock brake system (ABS) 24 is provided between the
fluid passages Pc, Qc and the fluid passages Pd, Pe; Qd, Qe.
[0032] A reaction force permission valve 25, which is a normally
closed solenoid valve, is connected between a fluid passage Ra
branching from the fluid passage Qa and a fluid passage. Rb. A
stroke simulator 26 is connected to the fluid passage Rb. The
stroke simulator 26 has a cylinder 27 and a piston 29 slidably
fitted in the cylinder 27 while being urged by a spring 28. A fluid
chamber 30, formed on the side of the piston 29 opposite from the
spring 28, communicates with the fluid passage Rb.
[0033] An actuator 51 of the slave cylinder 23 has an electric
motor 52, which may comprise a permanent magnet synchronous motor,
such as a brushless DC motor or an AC servo motor, a drive bevel
gear 53 provided on the rotating shaft of an electric motor 52, a
follower bevel gear 54 meshing with the drive bevel gear 53, and a
ball screw mechanism 55 operated by the follower bevel gear 54. A
sleeve 58 is rotatably supported in an actuator housing 56 via a
pair of ball bearings 57. An output shaft 59 is coaxially arranged
on an inner periphery of the sleeve 58. The follower bevel gear 54
is arranged on an outer periphery of the sleeve 58.
[0034] A pair of pistons 38A and 38B urged in a retreat direction
by a pair of return springs 37A and 37B are slidably disposed in a
cylinder body 36 of the slave cylinder 23. A pair of fluid pressure
chambers 39A and 39B are defined on the front faces of the pistons
38A and 38B, respectively. A front end of the output shaft 59 abuts
on a rear end of the rear piston 38A. One of the fluid pressure
chamber 39A communicates with the fluid passages Pb, Pc via ports
40A, 41A, while the other fluid pressure chamber 39B communicates
with the fluid passages Qb, Qc through ports 40B, 41B.
[0035] The structure of the ABS 24 is of a well-known type. The ABS
24 has two streams structurally identical to each other: a stream
including the disc brake devices 14 and 15 for braking the left
front wheel and the right rear wheel; and a stream for the disc
brake devices 18 and 19 for braking the right front wheel and the
left rear wheel. Of these streams, the stream for the disc brake
devices 14 and 15 will be described as a representative. It will be
understood that the stream for disk brake devices 18 and 19 works
in an identical fashion. A pair of in-valves 42 comprising normally
open solenoid valves are provided between the fluid passage Pc and
the fluid passages Pd, Pe. A pair of out-valves 44 comprising
normally closed solenoid valves are provided between the fluid
passages Pd, Pe on the downstream side of the in-valves 42 and a
reservoir 43. A fluid pressure pump 47 interposed between a pair of
check valves 45 and 46 is provided between the reservoir 43 and the
fluid passage Pc. The fluid pressure pump 47 is driven by an
electric motor 48.
[0036] As shown in FIG. 3, connected to an electronic control unit
U, are a fluid pressure sensor Sa for detecting the brake fluid
pressure generated by the master cylinder 11, a fluid pressure
sensor Sb for detecting the brake fluid pressure transmitted to the
disc brake devices 18 and 19, a vehicle wheel speed sensors Sc for
detecting the vehicle wheel speeds of the vehicle wheels, and a
motor rotational position sensor (resolver) Se for detecting the
rotational position of the electric motor 52, for controlling the
operation of the shutoff valves 22A and 22B, the reaction force
permission valve 25, the electric motor 52 of the slave cylinder 23
and the ABS 24.
[0037] As shown in FIG. 4, a field-weakening control section Ua of
the electronic control unit U comprises a target brake fluid
pressure calculator M1, a differentiator M2, a field current
calculator M3, and an electric motor controller M4. The fluid
pressure sensor Sa for detecting the brake fluid pressure generated
by the master cylinder 11 is connected to the target brake fluid
pressure calculator M1. The electric motor 52 of the slave cylinder
23 is connected to the electric motor controller M4.
[0038] The operation of an exemplary embodiment of the present
invention having the above-described arrangement will now be
described.
[0039] In a situation where the system operates normally, the
shutoff valves 22A and 22B, comprising normally open solenoid
valves, are demagnetized so as to be in an open state, and the
reaction force permission valve 25, comprising a normally closed
solenoid valve, is magnetized so as to be in an open state. In this
state, when the fluid pressure sensor Sa provided in the fluid
passage Qa detects a pushing force on the brake pedal 12 by the
driver, the actuator 51 of the slave cylinder 23 is activated. That
is, when the electric motor 52 is driven in one direction, the
output shaft 59 is advanced by the drive bevel gear 53, the
follower bevel gear 54 and the ball screw mechanism 55, so that the
pair of the pistons 38A and 38B urged by the output shaft 59 are
advanced. Because the ports 40A and 40B leading to the fluid
passages Pb and Qb are closed quickly after the pistons 38A and 38B
begin to advance, a brake fluid pressure is generated in the fluid
pressure chambers 39A and 39B. This brake fluid pressure is
transmitted to the wheel cylinders 16, 17, 20, and 21 of the disc
brake devices 14, 15, 18, and 19 through the opened in-valves 42 of
the ABS 24, thereby braking the vehicle wheels.
[0040] Because the ports 40A and 40B leading to the fluid passages
Pb and Qb are closed by the pistons 38A and 38B, the brake fluid
pressure generated by the master cylinder 11 is not transmitted to
the disc brake devices 14, 15, 18, and 19. At this time, the brake
fluid pressure generated in the other fluid pressure chamber 13B of
the master cylinder 11 is transmitted to the fluid chamber 30 of
the stroke simulator 26 through the opened reaction force
permission valve 25 to move the piston 29 against the spring 28,
thereby generating a pseudo pedal reaction force while permitting
the stroke of the brake pedal 12 to eliminate an uncomfortable
feeling to the driver.
[0041] The operation of the actuator 51 for the slave cylinder 23
is controlled so that the brake fluid pressure generated by the
slave cylinder 23 and detected by the fluid pressure sensor Sb
provided in the fluid passage Qc has a value corresponding to the
brake fluid pressure generated by the master cylinder 11 and
detected by the fluid pressure sensor Sa provided in the fluid
passage Qa, thereby generating the braking force in the disc brake
devices 14, 15, 18, and 19 according to the pushing force applied
to the brake pedal 12 by the driver.
[0042] If the slip ratio of any vehicle wheel is increased and a
tendency of locking is detected based on the output from the wheel
speed sensors Sc corresponding to the vehicle wheels during the
above-described braking, the ABS 24 is operated in a state in which
the slave cylinder 23 is maintained in the operating state, thereby
preventing locking of the vehicle wheels.
[0043] That is, when any vehicle wheel has a tendency of locking, a
pressure reducing operation is performed to release the brake fluid
pressure in the wheel cylinder by opening the out-valve 44 in a
state where the transmission of the brake fluid pressure from the
slave cylinder 23 is shut off by closing the in-valve 42
communicating with the wheel cylinder; and a pressure maintaining
operation is subsequently performed to maintain the brake fluid
pressure in the wheel cylinder by closing the out-valve 44, thereby
reducing the braking force to avoid locking of the vehicle
wheel.
[0044] When the vehicle wheel speed is recovered, thereby reducing
the slip ratio, a pressure increasing operation is performed to
increase the brake fluid pressure in the wheel cylinder by opening
the in-valve 42, thereby increasing the braking force for braking
the vehicle wheel. Each time the vehicle wheel has a tendency of
locking due to this pressure increasing operation, the
above-described pressure reducing, maintaining and increasing
operation is performed again. The operation is repeatedly performed
to generate the maximum braking force while preventing locking of
the vehicle wheels. The brake fluid flowing into the reservoir 43
during this process is returned by the fluid pressure pump 47 to
the fluid passages Pc and Qc on the upstream side.
[0045] During the above-described ABS control, the shutoff valves
22A and 22B are magnetized to be closed, thereby preventing a fluid
pressure fluctuation associated with the operation of the ABS 24
from being transmitted as a kickback from the master cylinder 11 to
the brake pedal 12.
[0046] If the slave cylinder 23 becomes inoperable due to power
failure or the like, the braking control is performed by the brake
fluid pressure generated by the mater cylinder 11 in place of the
brake fluid pressure generated by the slave cylinder 23.
[0047] That is, in the event of power failure or the like, as shown
in FIG. 2, the shutoff valves 22A and 22B, comprising normally open
solenoid valves, remain open; the reaction force permission valve
25 comprising a normally closed solenoid valve is automatically
closed; the in-valves 42, comprising normally open solenoid valves,
are automatically opened; and the out-valves 44, comprising
normally closed solenoid valves, are automatically closed. In this
state, the brake fluid pressure generated in the fluid pressure
chambers 13A and 13B of the master cylinder 11 passes the shutoff
valves 22A and 22B, the fluid pressure chambers 39A and 39B of the
slave cylinder 23 and the in-valves 42, without being absorbed by
the stroke simulator 26; and operates the wheel cylinders 16, 17,
20, and 21 of the disc brake devices 14, 15, 18, and 19,
respectively, for braking the vehicle wheels, thus generating the
braking force without any problem.
[0048] In such a case where the driver has suddenly applied a
pushing force to the brake pedal 12 in order to avoid a collision,
for example, it is required that the brake fluid pressure be
increased by activating the slave cylinder 23 as soon as possible.
However, there is a trade-off between the rated rotational speed
and rated torque of the electric motor 52. Therefore, if the rated
rotational speed of the electric motor 52 is set at a high value in
order to enhance the response of braking force generation in an
emergency, the rated torque decreases correspondingly, leading to a
possibility that the torque of the electric motor 52 becomes
insufficient under normal operating conditions. If both the rated
rotational speed and the rated torque of the electric motor 52 are
set at high values to prevent this possibility, this is
disadvantageous because the size and/or price of the electric motor
52 will be increased. Thus, in the embodiments of the present
invention, the field-weakening control of the electric motor 52 is
performed to increase the rotational speed in an emergency
situation where the response of braking force generation is
required to be enhanced.
[0049] That is, in the field-weakening control section Ua of the
electronic control unit U shown in FIG. 4, the target brake fluid
pressure calculator M1 to which the brake fluid pressure of the
master cylinder 11, detected by the fluid pressure sensor Sa is
inputted, calculates a target brake fluid pressure Pref
corresponding to the brake fluid pressure of the master cylinder
11, i.e., the brake fluid pressure to be generated in the slave
cylinder 23. The differentiator M2 calculates a rate of change
dPref/dt of the target brake fluid pressure by time-differentiating
the target brake pressure Pref. A situation where the rate of
change dPref/dt of the target brake fluid pressure is high is, for
example, the case where the driver has suddenly applied a pushing
force to the brake pedal 12, corresponding to an emergency where a
quick building-up of a braking force is required. Such a high rate
of change of the target brake fluid pressure can be readily
determined, for example, when the rate of change exceeds a
predetermined value.
[0050] The electric motor 52, which may comprise a permanent magnet
synchronous motor, such as a brushless DC motor or an AC servo
motor, is controlled on the basis of an excitation current command
value Iq* of a q-axis component and an excitation current command
value Id* of a d-axis component. The excitation current command
value Iq* of a q-axis component is outputted in proportion to a
torque command value to be generated in the electric motor 52. The
excitation current command value Id* of a d-axis component is
basically 0 when the field-weakening control of the electric motor
52 is not performed. When the excitation current command value Id*
of a d-axis component is increased in the negative direction, the
amount of field-weakening increases and the rotational speed of the
electric motor 52 increases.
[0051] The field current calculator M3 calculates a field current
command value Id* by, for example, searching the map shown in FIG.
5 using the rate of change dPref/dt of the target brake fluid
pressure as a parameter. The electric motor controller M4 performs
the field-weakening control of the electric motor 52 on the basis
of the calculated field current command value Id*. At this time,
the field current command value Id* increases in the negative
direction with an increase in the rate of change dPref/dt of the
target brake fluid pressure, and correspondingly the amount of
field-weakening increases, thereby increasing the rotational speed
of the electric motor 52. Therefore, in an emergency situation
where the rate of change dPref/dt of the target brake fluid
pressure increases, as shown in FIG. 6, the field current command
value Id* is increased in the negative direction, thereby
increasing the amount of field-weakening of the electric motor 52
to increase the rotational speed of the electric motor 52, so that
the slave cylinder 23 is quickly activated. Thus, it is possible to
enhance the response of braking force generation.
[0052] As described above, while using the electric motor 52 which
does not have a particularly high rated rotational speed, it is
possible to enhance the response of braking force generation in an
emergency situation requiring an enhanced response by performing
the field-weakening control of the electric motor 52 to increase
the rotational speed thereof.
[0053] FIGS. 7A to 10 show a second embodiment of the present
invention.
[0054] In the first embodiment, the field-weakening operation of
the electric motor 52 is performed in accordance with the target
brake fluid pressure, whereas in the second embodiment, the
field-weakening operation of the electric motor 52 is performed in
accordance with the load characteristics of the slave cylinder 23
represented by caliper rigidity of the wheel cylinders 16, 17, 20,
21.
[0055] The calipers of the wheel cylinders 16, 17, 20, 21 generate
a braking force by pressing the brake pad against the brake disc
with the brake fluid pressure. In the case where the rigidity of
the caliper main body supporting the brake pad decreases due to
secular change or the brake pad itself wears, even if the brake
fluid pressure is applied on the caliper, the contact face pressure
between the brake pad and the brake disc does not quickly increase,
leading to a problem of a lowered response of braking force
generation.
[0056] When the driver applies a pushing force to the brake pedal
12 as shown in FIG. 7A, there is built-up a target brake fluid
pressure to be generated by the slave cylinder 23 corresponding to
the pushing force applied to the brake 12, i.e., a target brake
fluid pressure to be supplied to the caliper as shown in FIGS. 7B
and 7C. However, even if the slave cylinder 23 generates a brake
fluid pressure exactly equal to the target brake fluid pressure, an
actual fluid pressure of the caliper is built-up with a delay in
response time a, b, c which varies depending on the degree of
rigidity of the caliper main body and the degree of wear of the
brake pad, as shown in FIG. 7B.
[0057] As shown in FIG. 7C, in the second embodiment, the
field-weakening control of the electric motor 52 is performed such
that the field current command value Id* is increased with an
increase of response time a, b, c to increase the rotational speed
thereof, thereby minimizing the delay in the response time by
making the delay in the response time constant.
[0058] The operation of the field-weakening control according to
the second embodiment will be described with reference to the
flowchart of FIG. 8.
[0059] First, when an ignition switch is turned on at Step S1, the
electric motor 52 is automatically operated at Step S2, even if the
driver does not apply a pushing force to the brake pedal 12, to
generate a brake fluid pressure in the slave cylinder 23, and the
load characteristics of the slave cylinder 23 at the time of
operation of the electric motor 52 are detected at Step S3.
[0060] The load characteristics of the slave cylinder 23 are
characteristics of change in the caliper fluid pressure with
respect to the rotational position of the electric motor 52, i.e.,
the advance positions of the pistons 38A, 38B. The rotational
position of the electric motor 52 is detected by the motor
rotational position sensor (resolver) Sd (see FIG. 3). The caliper
fluid pressure is detected by the fluid pressure sensor Sb (see
FIGS. 1 and 2) that is provided in the fluid path Qc.
[0061] FIG. 9 is an exemplary graph showing a technique of
detecting load characteristics based on detected caliper fluid
pressure and detected motor rotational position. The electric motor
52 is driven, the caliper fluid pressure in each rotational
position is detected, and a rotational position Ang1 of the
electric motor 52, which is obtained when a predetermined caliper
fluid pressure P is obtained, is stored. Because the load
characteristics change depending on the degree of rigidity of the
caliper main body and the degree of wear of the brake pad, the
rotational position Ang1 of the electric motor 52 corresponding to
the predetermined caliper fluid pressure P changes corresponding to
secular change of the slave cylinder 23.
[0062] Referring again to the flowchart of FIG. 8, at Step S4, the
appropriate field current command value Id* for the field-weakening
control of the electric motor 52 is searched for in the map of FIG.
10. As shown in FIG. 10, the field current command value Id*
increases in the negative direction with increase in the rotational
position Ang1, Ang2, Ang3 of the electric motor 52, i.e., with
progress in the advance position of the pistons 38A, 38B, and
correspondingly the amount of field-weakening increases to increase
the rotational speed of the electric motor 52. Therefore, if a
pushing force is applied to the brake pedal 12 at Step S5 to
activate the slave cylinder 23 and the field-weakening control of
the electric motor 52 is performed at Step S6, the amount of
field-weakening of the electric motor 52 is increased in proportion
to the decrease in the caliper rigidity and in proportion to the
increase of the wear of the brake pad to thereby increase the
rotational speed of the electric motor 52. Therefore, the slave
cylinder 23 is quickly activated to compensate for the
deterioration of the response of braking force generation.
[0063] Because the detection of the load characteristics of the
slave cylinder 23 is performed when the ignition switch is turned
on, it is possible to perform an appropriate field-weakening
control by detecting the latest load characteristics each time the
engine is started.
[0064] Exemplary embodiments of the present invention have been
described above, but various changes in design may be made without
departing from the subject matter of the present invention.
[0065] As one example, in the first and second embodiments
described above the field weakening control is performed based on
either a rate of change of a target braking force set in accordance
with an operational amount of braking operation by a driver or on
detected load characteristics of the electric braking force
generator. However, it is certainly possible to perform the filed
weakening control based on both the detected load characteristics
of the electric braking force generator and the rate of change of
the target braking force.
[0066] As a second example, the electric braking force generator of
the present invention is not limited to the slave cylinder 23 of
the embodiments. The electric braking force generator may be of a
mechanical type (non-fluid-pressure type) that generates a braking
force by directly driving a brake pad through the electric motor
52.
[0067] As another example, in the second embodiment, the load
characteristics of the slave cylinder 23 are detected as the
relationship of the caliper fluid pressure to the rotational
position of the electric motor 52. However, because the rotational
position of the electric motor 52 and the advance positions of the
pistons 38A, 38B are in a certain corresponding relationship, the
load characteristics of the slave cylinder 23 may be detected as
the relationship of the caliper fluid pressure to the advance
position of the pistons 38A, 38B. The advance positions of the
pistons 38A, 38B can be detected by an appropriate position
sensor.
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